Capacitive key touch sensing using analog inputs and digital outputs

• US 8,310,381 B2
• Filed: 06/04/2010
• Issued: 11/13/2012
• Est. Priority Date: 09/28/2009
• Status: Active Grant

• Current Assignee: Neodrón Limited (Atlantic IP Services Limited)
• Original Assignee: Microchip Technology Incorporated
• Inventors: Lin, Chien-Yi
• Primary Examiner(s): Nguyen, Khai M
• Application Number:
  US12/794,153
• Publication Number:
  US 20110074609A1
• Time in Patent Office: 893 Days
• Field of Search: 341/22, 341/33, 341/34, 178/18.06, 178/18.07, 345/168, 345/173
• US Class Current: 341/33
• CPC Class Codes: G06F 3/044 by capacitive means
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 17/9622 using a plurality of detectors, e.g. keyboard
• CPC Class Codes: H03K 2017/9613 using two electrodes per touch switch
• CPC Class Codes: H03K 2217/96073 Amplitude comparison

Abstract

A pulse is applied through a capacitive touch key sensor to a sampling capacitor of an analog-to-digital converter (ADC). The voltage charge arriving at the sampling capacitor will be maximum when there is substantially no shunt capacitance between the capacitive touch key sensor and the sampling capacitor. However, a object such as an operator'"'"'s finger when in close proximity to the capacitive touch key sensor will create a capacitive shunt to ground that diverts some of the charge that is supposed to go to the sampling capacitor and thereby reduces the voltage charge on the sampling capacitor. This change in charge voltage when the capacitive touch key sensor is activated (touched) may be easily detected with the ADC. In addition, light emitting diodes (LEDs) may be integrated with the capacitive touch key sensors and use the same connections on an integrated circuit device in a time division multiplexed manor.

Claims

1. An integrated circuit device that senses actuation of a capacitive touch key, comprising:

an analog-to-digital converter (ADC) having a sampling capacitor, wherein the ADC converts an analog voltage on the sampling capacitor to a digital representation thereof;

a digital processor coupled to a digital output of the ADC;

a switch matrix controlled by the digital processor;

a digital driver controlled by the digital processor;

a first node adapted for coupling to a capacitive touch key, the first node is coupled to the sampling capacitor through the switch matrix;

a second node adapted for coupling to the capacitive touch key, the second node is coupled to the digital driver through the switch matrix;

wherein a voltage pulse from the digital driver and through the capacitive touch key charges the sampling capacitor to a first voltage in proportion to a capacitance ratio thereof,

wherein when an object having a capacitance to a common potential is in close proximity to the capacitive touch sensor some of the voltage pulse through the capacitive touch sensor is diverted from the sampling capacitor to the common potential, and

whereby the sampling capacitor is charged to a second voltage when the object is in close proximity to the capacitive touch key, wherein the second voltage is less than the first voltage.

2. The integrated circuit device according to claim 1, wherein the object is a finger.

3. The integrated circuit device according to claim 1, wherein the common potential is substantially ground potential.

4. The integrated circuit device according to claim 1, wherein the digital processor stores in a memory thereof digital representations of voltages from the ADC, when at least one of the digital representations of voltages stored in memory is less than the others, a signal is generated indicating that the capacitive touch key was actuated.

5. The integrated circuit device according to claim 1, further comprising a light emitting diode (LED) and a high speed switching diode connected in series, and the series connected LED and high speed switching diode are coupled in parallel with the capacitive touch key, wherein the LED is turned on when the switch matrix couples the first node to a logic high and the second node to a logic low.

6. The integrated circuit device according to claim 5, wherein the voltage pulse from the digital driver is generated and the LED is turned on during alternate time periods.

7. The integrated circuit device according to claim 5, wherein the capacitive touch key and the LED use common connections.

8. The integrated circuit device according to claim 1, wherein the integrated circuit device is a mixed signal microcontroller.

9. A system for sensing actuation of one or more of a plurality of capacitive touch keys, said system comprising:

a plurality of capacitive touch keys,

an integrated circuit device having a plurality of external connections coupled to the plurality of capacitive touch keys, the integrated circuit device comprising;

an analog-to-digital converter (ADC) having a sampling capacitor, wherein the ADC converts an analog voltage on the sampling capacitor to a digital representation thereof;

a digital processor coupled to a digital output of the ADC;

a switch matrix controlled by the digital processor, wherein the plurality of external connections are coupled to the switch matrix;

a digital driver controlled by the digital processor, wherein the digital driver is coupled to the switch matrix;

wherein a voltage pulse from the digital driver is coupled to a selected one of the plurality of capacitive touch keys through the switch matrix and a resulting voltage from the selected one of the plurality of capacitive touch keys charges the sampling capacitor to a first voltage in proportion to a capacitance ratio thereof,

wherein when an object having a capacitance to a common potential is in close proximity to the selected one of the plurality of capacitive touch keys some of the voltage pulse through the selected one of the plurality of capacitive touch keys is diverted from the sampling capacitor to the common potential, and

whereby the sampling capacitor is charged to a second voltage when the object is in close proximity to the capacitive touch key, wherein the second voltage is less than the first voltage.

10. The system according to claim 9, wherein the object is a finger.

11. The system according to claim 9, wherein the common potential is substantially ground potential.

12. The system according to claim 9, wherein the digital processor stores in a memory thereof digital representations of voltages from the ADC for each one of the plurality of capacitive touch keys, wherein when a digital representation of a voltage sampled by the ADC is less then a respective one of the stored voltage representations in memory, a signal is generated indicating that an associated one of the plurality of capacitive touch keys was actuated.

13. The system according to claim 9, further comprising a plurality of light emitting diodes (LEDs) and a plurality of high speed switching diodes, wherein a selected one of the plurality of LEDs is turned on when the switch matrix couples logic high and logic low levels to the selected one of the plurality of LEDs so as to cause current to flow therethrough, wherein the plurality of light emitting diodes (LEDs) are connected is series with respective ones of the plurality of high speed switching diodes and the series connected plurality of LEDs and high speed switching diodes are coupled in parallel with respective ones of the plurality of capacitive touch keys.

14. The system according to claim 13, wherein the voltage pulse from the digital driver is generated and the LED is turned on during alternate time periods.

15. The system according to claim 13, wherein the plurality of capacitive touch sensors and respective ones of the plurality of LEDs are coupled to the same external connections.

16. The system according to claim 9, wherein the integrated circuit device is a mixed signal microcontroller.

17. A method for sensing actuation of any one of a plurality of capacitive touch keys arranged in P(x) rows and A(y) columns, said method comprising the steps of:

setting P(x) rows and A(y) columns to a logic low for discharging any residual charges on a plurality of capacitive touch keys;

coupling each of the A(y) columns to an analog input;

generating a pulse on each of the P(x) rows for each of the A(y) columns;

measuring an analog voltage associated with each of the plurality of capacitive touch keys that are generated by each of the pulses;

storing the measured analog voltages associated with each of the plurality of capacitive touch keys;

determining when any one of the stored measured analog voltages is below a certain voltage threshold; and

indicating which one of the plurality of capacitive touch keys has the associated stored analog voltage below the certain voltage threshold.

18. The method according to claim 17, wherein the step of measuring the analog voltages is done with an analog-to-digital converter (ADC) that converts each of the measured analog voltages into digital voltage values.

19. The method according to claim 18, wherein the digital voltage values are stored in a memory.

20. The method according to claim 19, wherein the step of determining when any one of the stored measured analog voltages is below a certain voltage threshold is done with a microcontroller and the digital voltage values stored in the memory.